1. Field of the Invention
The present invention concerns force sensors and in particular sensors in which variations in a beam's natural resonant frequency caused by tension applied to the beam are used to measure the tension.
2. Description of Related Art
Vibrating beam force sensors are known and essentially comprise a beam driven into flexural vibration by piezoelectric transducers mounted at its ends. The vibrational frequency changes in response to changes in the tension of the beam and these changes are measured to provide a measurement of the applied tension.
One type of vibrating beam force sensor is described in British Pat. Specification No.2141231B and comprises a sensor unit having in effect three parallel beams of piezoelectric material and of different widths the outer pair of which flex in antiphase to the inner beam. Such an arrangement has an improved Q factor when compared to a sensor employing a single beam. The Q factor indicates the amount of energy locked into the vibrating structure relative to the amount of energy required to maintain the vibrations. Another advantage of three-beam arrangements is a greater degree of isolation from the effects of small changes in the mounting conditions. However piezoelectric materials are expensive and difficult to machine.